The discovery of passive mode-locking via saturable absorbers has led to optical femto-second pulses with applications ranging from eye-surgery to the analysis of chemical reactions on ultra-short timescales. In the frequency domain, a train of such optical pulses corresponds to a frequency comb (equidistant optical laser lines spaced by the pulse repetition rate, see e. g. R. Holzwarth et al. in “Physical Review Letters” vol. 85, 2000, p. 2264-2267), and they find use e. g. in precision spectroscopy and optical frequency metrology.
Not relying on mode-locking, frequency combs can also be generated in continuously driven, high-quality-factor Kerr-nonlinear optical microresonators (see e. g. T. J. Kippenberg et al. in “Science” vol. 332, 2011, p. 555-559).
Frequency comb generation in optical microresonators is achieved via cascaded-four-wave mixing (FWM) mediated by the Kerr-nonlinearity of the resonator material. A continuous-wave (CW) pump laser is converted into equally spaced optical modes, where the mode spacing corresponds to the freespectral range (FSR), or equivalently, the inverse resonator roundtrip time 1/TR. Various materials and geometries of optical microresonators, in particular optical microresonators with fully planar CMOS compatible design, are available and advances have been made with the demonstration of octave spanning spectra, phase stabilization, low phase noise microwave generation, and arbitrary optical waveform generation.
Nevertheless, the conventional generation of frequency combs still suffers from the following substantial problem. While the four-wave mixing process results in coupled phase relations between all the optical modes and correspondingly in the generation of a periodic time domain output, this however does not correspond intrinsically to pulses. Pulse shaped waveforms in time domain have been mentioned as a potential output of a conventional optical microresonator (see EP 1 988 425 A1). However, a mode-locking mechanism has not been disclosed in EP 1 988 425 A1, and the frequency combs conventionally created with practical resonators typically do not have a pulse shape.
Mode-locking, i.e. full phase synchronization inside the microresonators would solve the above problem. Moreover, mode-locking would provide a way to achieve low-noise frequency comb spectra, with little line-to-line power variation and without spectral gaps—a goal difficult to reach in systems with mode-spacings below 100 GHz, where multiple, inconsistent subcombs may form. While mode-locking has been considered possible for microresonators (A. Matsko et al., Optics Letters 36, 15, 2845-2847, 2011), a mode-locking mechanism for optical microresonators has not yet been shown. Introducing a saturable absorber or an equivalent optical element into the laser cavity, easily possible in conventional laser systems, would impair the optical quality factor Q of the microresonator. Approaches with other mode-locking mechanisms based on self-focusing or small saturable components of material absorption were not successful.
A. Gasch et al. (in “Applied Physics Letters” vol. 44, 1984, p. 1105-1107) have described soliton modes in nonlinear microwave resonators. It has also been proposed that Kerr-cavity solitons may form in optical microresonators (see F. Leo et al. in “Nature Photonics” vol. 4(7), 2010, p. 471-476). This soliton formation could lead to mode-locking. However, to date no definitive proof nor practical mechanism of optical mode locking has been identified.